Proper needle thread stitch formation is a necessity in generating high-quality sewn products. Improper needle thread stitch formation diminishes the durability and functionality of a sewn seam. Improper needle thread stitch formation is the result of skipped stitches, thread breaks, absence of needle threads, needle breaks, and unpicked-up needle thread.
Sewn product manufacturers pay high costs to sample in-process sewn products and to inspect finished products for needle thread stitch formation defects. These defective stitches must be repaired, if detected before the products are shipped, to provide first-quality products to customers. The whole process of detecting and correcting these defective stitches contributes to a loss of productivity. A reliable, low maintenance, low cost apparatus and method for detecting improper needle thread stitch formation on single and multiple needle sewing machines at all stitch formation rates has not been provided to the sewn products industry.
The prior art abounds with various types of sewing machine monitoring apparatuses. Such monitoring apparatuses typically include mechanisms for stopping a sewing machine, or at least providing a warning signal, when thread breakage or other disturbance of the feed of the thread is detected.
U.S. Pat. No. 4,841,890, issued on Jun. 27, 1989, to Tancs, entitled "Thread Break Indicating Device For Sewing Machines Using a Photoamplifier," monitors the influence of unbroken sewing thread on a light beam during sewing machine operation to detect a needle thread break. The device detects thread breaks and has no capacity to reliably detect proper stitch formation. This deficiency is a result of thread movement of unbroken thread in a sewing machine even when stitch formation does not occur. The device has no capacity to detect thread movement at a particular time during the stitch formation cycle.
U.S. Pat. No. 3,587,497, issued on Jun. 28, 1971, to Beazley, entitled "Thread Breakage Detector," discloses apparatus which monitors check spring contact with a fixed contact block to detect a needle thread break. This apparatus group detects thread breaks and has no capacity to reliably detect proper stitch formation. This deficiency is a result of thread movement of unbroken thread in a sewing machine cycle causing check spring movement when stitch formation does not occur. The device has no capacity to detect thread movement at a particular time during the stitch formation cycle and does not allow for equipment adjustments to expected extremes for all applications of sewing. This adjustment limitation is a result of the fixed block sensor positioning. This device is limited to detecting needle thread breaks on machines producing federal stitch standard 301 (lock stitch) type stitches.
U.S. Pat. No. 4,429,651, issued on Feb. 7, 1984, and entitled "Device For Detecting Absence of a Thread in a Sewing Machine," discloses a device which detects the absence of needle thread or bobbin thread in a sewing machine. The device detects the absence of thread and includes a piezoelectric element placed under the throat plate of the sewing machine. This device depends on high cost, high maintenance piezoelectric sensor technology with inherent, highly sensitive calibration requirements. This device is limited to a machine with unencumbered space to permit attachment of a piezoelectric sensor under the throat plate of the sewing machine. This monitor is limited to a portion of machines capable of sewing federal stitch standard type 301 stitches.
U.S. Pat. No. 4,754,722, issued July 1988 to Rohr et al., for "Thread Break Detector For a Sewing Machine," discloses apparatus including a sensor which detects for thread breaks by monitoring for the absence of sewing thread between two thread guides. The uppermost thread guide is secured to a stationary support, and the lowermost thread guide is secured to the needle bar (needle holer) for vertical movement therewith, and the sewing thread is threaded through these guides and the needle eye. This apparatus requires that existing machines be modified to receive these two guides. The needle bar must be substantially modified to receive the lower guide. Additionally, the addition of the movable guide forces the thread to deviate from a normal single line path during proper machine operation to a plurality of paths that are within a specific range. The apparatus of the present invention does not require the addition of guide assemblies to existing machines, and, particularly, the present invention does not require that the needle bar be modified to receive a guide assembly which must be vertically movable with the needle.
U.S. Pat. No. 4,938,159, issued Jul. 3, 1990, to Yoshio Shibata for "Thread Detecting Apparatus in a Sewing Machine," discloses apparatus for detecting needle thread breaks. The Shibata apparatus includes a first AND gate which receives signals from a timing signal generator means which includes a detecting member on the main drive shaft of the machine, a proximity sensor which utilizes magnetism or electrostatic capacity, and a first gate forming circuit. The proximity sensor senses every time the detection member is rotated one complete rotation. A thread detecting sensor is provided for detecting the end of the thread or thread break (absence of thread) and for transmitting an electrical indication thereof to a second AND gate circuit. A comparator circuit must be provided to compare the two signals and provide a comparator generated signal in response to signals from both AND gates, and, if the comparator generated signal exceeds a reference value, the machine operation may be terminated. It should be noted that the thread sensor circuit of Shibata is dependent on the use of an electrically charged thread engages the sensor element which is mounted in the very confined space beneath the throat plate.
Each of the above patents are directed to apparatuses which are designed to monitor the needle thread for needle thread breakage only. Thread breakage typically occurs for the following reasons:
1. Misaligned offwinding from the thread package. PA0 2. Trapping at the thread package base. PA0 3. Thread trapped at thread guide. PA0 4. Snarling before tension disc. PA0 5. Excessive tension. PA0 6. Broken check spring. PA0 7. Sharp edges on throat plate, hook point, needle guard, bobbin case, needle groove, or eye. PA0 8. Thread fraying at needle. PA0 9. Excessive needle heat, groove, or eye blocked with melted fabric. PA0 10. Hook overheating. PA0 11. Poor quality thread. PA0 1. Badly worn thread on the bobbin. PA0 2. Tension too tight or bobbin overrunning. PA0 3. Sharp edges on bobbin case or spring or looper eyelet. PA0 4. Bobbin case not fitting correctly. PA0 1. Failure of hook, looper, or needle to enter thread loops at the correct time. PA0 2. Thread loop failure due to incorrect needle size/style for thread size/type. PA0 3. Thread loop failure due to incorrect setting of thread control mechanism causing thread loop starvation. PA0 4. Flagging of fabric due to poor presser foot control or too large a throat plate hole. PA0 5. Needle deflections or bent needle. PA0 6. Incorrect sewing tension in the needle or underthreads. PA0 7. Poor thread loop formation.
U.S. Pat. No. 4,602,582, issued on Jul. 29, 1986, to James W. Rawson for "Monitoring Looper Thread Feed Monitoring Device in a Sewing Machine," is directed to apparatus for monitoring the looper thread instead of needle thread. The apparatus includes a source of light which is conveyed into the thread feed region of the sewing machine by fiber optics. Light receiving means is provided beneath the throat plate to receive the light and produce a monitor signal representative of the received light. A timing means is provided which is responsive to the monitor signal to produce a timing window during which a signal is present if the looper thread is being correctly fed to the sewing machine. The apparatus requires that the end of the fiber optic be in the confined space beneath the throat plate and that the light receiving means also be positioned in this confined area which is subjected to the gathering of dust and, particularly, lint from the sewn material. The presence of such dust and lint can cause the light to be obstructed in the looper area and is, therefore, susceptible to causing the light receiving means to provide false readings. The device of the patent to Rawson monitors the looper thread and not the needle thread, as does the device of the present invention. Some causes of looper thread abnormalities are as follows:
A sewing machine typically includes a needle which receives thread through its eye from a source of thread which may be mounted on the body of the sewing machine or remotely therefrom. The thread generally follows a path through various thread guides on the machine, through a thread tensioning device, a thread take-up device, and then through other guide means mounted above the needle. The thread is then directed through the eye of the needle. The take-up device pulls the thread tight between the needle and the thread tensioning device.
Such sewing machines typically include a needle thread gathering cycle and a stitch formation cycle. During the thread gathering cycle, the needle gathers thread for use during the stitch formation cycle. During the cycle of stitch formation, needle thread forms a loop at the eye of the needle, and needle thread is caught and the loop is enlarged by a needle thread loop handling device such as a looper, a bobbin hook assembly, or a spreader mounted beneath the throat plate needle hole if a stitch is properly formed. During this stitch formation cycle, a portion of the cycle contains this loop enlargement and the thread above the needle in the sewing machine will react in a predictable pattern between the tension device (typically mounted on the machine body above the needle and through which the thread is threaded) and the needle. If the stitch is being formed properly, the thread will move in a predetermined path between the tension device and the needle. In this predetermined path, the thread assumes a position which places it in registry with a detector mounted on the machine between the take-up device and the needle.
On some sewing machines (such as 301 stitch type machines), the tensioning device includes a check spring assembly having a pivoted arm provided with an eyelet through which the thread is threaded. The thread is then directed through the take-up device prior to being directed through the needle eye. The pivotally mounted arm is pivoted as a result of the needle moving the tensioned thread through the eyelet on an arm of the check spring. Pivotal movement (or non-movement) of the arm can be detected by sensors to generate electrical signals which indicate proper (or improper) arm positions. The arm position is caused to change in response to non-linear (transverse) positioned changes in the thread.
It is to be understood that during the stitch formation cycle, if the thread loop is not "caught" by the hook assembly (or looper) mounted beneath the throat plate, improper thread positioning will occur and be sensed by the sensors discussed above.
It is to be further understood that the thread monitoring system of the present invention monitors the needle thread (not the looper thread) to detect if there is improper conditions at the needle thread loop handling device (looper, bobbin, or spreader, etc.) which would cause a skipped stitch condition. Such skipped stitches typically occur as a result of the following abnormalities:
The monitoring system of the present invention will provide a warning or will shut down machine operations if a skipped stitch attributable to any of the above-enumerated abnormalities occur.
Accordingly, it is an object of the present invention to provide a method and apparatus for detecting improperly formed needle thread stitches.
It is another object of the present invention to provide a method and apparatus for detecting the presence of such improperly formed needle thread stitches on all type stitch formations, including at least U.S. standard types 101 (chain stitch), 301 (lock stitch), and 401 (double lock chain stitch).
In accomplishing the recited objects, the needle thread stitch formation monitor of the present invention includes a high-speed synchronizer sensor attached to the body of the sewing machine for detecting a sensing medium provided on the main shaft when the thread take-up mechanism of the machine is in the particular portion of the revolution of the shaft of the machine during a stitch formation cycle. In one embodiment of the present invention, the needle thread stitch formation monitor includes a high-speed thread sensor which is actuated by the thread during the stitch formation cycle to provide an output indicative of proper or improper thread position relative to the thread sensor. The thread sensor is a light emitter and light detector assembly positioned between the tension device and the needle, for detecting the presence of needle thread at a particular position between the tension device and the needle on all type stitch formations. In another embodiment of the present invention, the movement of the tension spring of the check spring device, such as is provided on 301 stitch type machines, for example, is detected by a proximity sensor. The sensor is actuated to produce a predetermined output as a result of the thread displacing (or not displacing) an arm of the check spring device during the stitch formation cycle. In each of the above embodiments, the relative timing of the signals from the high-speed thread sensors during the cycle is established by a control circuit including an AND gate which compares signals from the high-speed synchronizer sensor and the high-speed thread sensors. The control circuit determines when a stitch is being properly or improperly formed. When an improperly formed stitch is detected, the control circuit provides a relay output which is used to activate other devices to stop the machine, delay the cycle of the machine, count the occurrences of improperly formed stitches, and alert a sewing machine operator through various means of the absence of a properly formed stitch.
Mounting the high-speed needle thread detecting sensor on the body of the sewing machine between the take-up device and the needle provides a capability for more than one needle tread monitoring sensor per machine to thus allow multiple needle thread monitoring on one machine, if desired. The position of the sensor above the needle of the sewing machine allows the use of optic sensing mechanisms without a loss of function due to lint produced under the throat plate.